best answer gets branlyiest and 100 points! (i really need help!)

The splendor falls on castle walls
And snowy summits old in story;
The long light shakes across the lakes,
And the wild cataract leaps in glory.
5 Blow, bugle, blow, set the wild echoes flying;
Blow, bugle; answer, echoes, dying, dying, dying.
O, hark, O hear! how thin and clear,
And thinner, clearer, farther going!
O, sweet and far, from cliff and scar,
10 The horns of Elfland faintly blowing!
Blow, let us hear the purple glens replying.
Blow, bugle; answer, echoes, dying, dying, dying.

1. Which lines are the refrain?

2. How does the refrain contribute to the tone of the poem?